1. Field of the Invention
The present invention relates to an indirect transfer type image recording apparatus and an indirect transfer type image recording method in which a toner image formed on an image carrier is transferred to a recording medium or an intermediate transferor so as to record an image; and relates to an image carrier for use in such an image recording apparatus and such an image recording method. Particularly, the present invention relates to an image recording apparatus and an image recording method using electrophotographically recording technology, electrostatically recording technology, ionography, magnetography, etc., and an image carrier for use in such an image recording apparatus and such an image recording method.
2. Description of the Related Art
Conventionally, in such an indirect transfer type image recording technique, differently from an ink-jet printer or the like in which an image is recorded directly on a recording medium such as recording paper or the like, a toner image is first formed on an image carrier. The toner image formed on the image carrier is transferred to recording paper or the like. The transferred toner image is fixed to obtain a recorded image. Then, after the toner image is transferred to the recording medium such as recording paper or the like, residual toner on the image carrier is generally recovered and abolished by cleaning means. For example, in an electrophotographic image recording apparatus, an image is recorded by a charging step, an exposing step, a developing step, a transferring step, a fixing step, and a cleaning step. In the charging step, the surface of an image carrier having a photosensitive layer in the surface is charged uniformly. In the exposing step, the charged surface of the image carrier is irradiated with image light so that an electrostatic latent image is formed. In the developing step, toner is made to adhere to the electrostatic latent image so as to form a toner image. In the transferring step, the toner image is transferred to a recording medium. In the fixing step, the toner image on the recording medium is fixed. In the cleaning step, residual toner on the image carrier in the transferring step is removed. In the cleaning step of these steps, a rubber blade or a brush having flexibility is pressed against the surface of the image carrier so as to recover the residual toner on the surface of the image carrier, and the recovered toner is accumulated in a recovery vessel and abolished periodically.
In such an image recording apparatus, it is necessary to always detect or measure the quantity of recovered toner accumulated in the recovery vessel so as to abolish the toner or exchange the recovery vessel before the recovery vessel is full of the toner. In addition, if the image recording apparatus is miniaturized so that a large space for installing the recovery vessel cannot be ensured, the inside of a drum-like image carrier may be used as a space for recovering toner. In such a case, the time to exchange the image carrier has to be established in accordance with the quantity of recovered toner. Further, reuse of recovered toner has proceeded from the point of view of environmental protection. However, the reuse of recovered toner involves a lot of problems such as a problem of separation, a problem of energy for conveyance and reuse, a problem of a recovery method and a storage place, and so on.
As means to solve such problems, the following means have been considered. (1) First means is to improve the transfer efficiency with which a toner image is transferred to a recording medium. If the transfer efficiency to the recording medium is improved, residual toner on the image carrier is reduced correspondingly. Thus, the quantity of toner to be recovered and disposed of is also reduced. (2) Second means is to recover residual toner from the image carrier and return the residual toner to the developing means so as to reuse the toner for development. If all the recovered toner is reused, it is not necessary to abolish the toner. (3) Third means is to solve disadvantage caused by residual toner without cleaning the image carrier. Thus, it is not necessary to recover the residual toner by cleaning, and it is expected that waste toner is prevented from being produced.
As the first means (1) to improve the transfer efficiency, the following techniques have been proposed.
(a) In the technique disclosed in JP-A-56-126872, the area where an electric field is formed for transfer is increased to improve the transfer efficiency.
(b) In the technique disclosed in JP-A-58-88770 or JP-A-58-140769, an alternating electric field is formed in a transfer position. This alternating electric field applies force to toner on an image carrier so as to vibrate the toner. Thus, separation of the toner from the image carrier is accelerated.
(c) In the technique disclosed in JP-A-52-126230, ultrasonic waves are radiated to an image carrier in a transfer position so as to generate vibrations. Thus, the adhesive force of toner grains is reduced.
(d) In the technique disclosed in JP-A-2-1870, JP-A-2-81053, JP-A-2-118671, JP-A-2-118672, or JP-A-2-157766, strippable particulates of silica or the like are contained in a developer. The particulates are interposed between toner and a photosensitive body. Thus, the adhesive force between the toner and the photosensitive body is reduced so that the transfer efficiency of the toner is enhanced.
(e) In the technique disclosed in JP-A-1-134485, transparent and colorless toner is made to adhere to a latent image formed on an image carrier, and colored toner is further put thereon to adhere thereto. Then, development is carried out. In a toner image formed thus, approximately 100% of the colored toner is transferred.
Although each of the disclosed techniques (a), (b) and (c) shows the effect of improvement of the transfer efficiency, a certain degree of toner still stays behind in the image carrier after transfer. Therefore, those techniques are not satisfactory to reduce waste toner.
On the other hand, in the disclosed technique (d), it is necessary to add an adequate quantity of strippable particulates to the developer, and coat toner with the strippable particulates uniformly. Actually, however, it is difficult to coat all the toner with the strippable particulates uniformly. It is therefore impossible to eradicate the existence of toner coated insufficiently. In addition, even if all the toner is coated with the strippable particulates uniformly, the strippable particulates are free from the toner due to various stresses such as stirring, layer thickness regulation, and so on, given thereto in a developing machine. Therefore, to keep the toner coated with the strippable particulates uniformly, it is necessary to realize a developing machine which gives no stress to the toner. Further, since it is necessary to add a large quantity of strippable particulates, the strippable particulates may adhere to the toner surface or the carrier surface due to use for a long term so that the electification property of the developer may be degraded, or the free strippable particulates may be flocculated. Thus, the flowability of the developer may be lowered to cause uneven development. In addition, the toner to which a large quantity of strippable particulates have been added is rich in flowability so that a toner image is disarranged easily when the toner image abuts against a recording medium at the time of transfer. Thus, such a phenomenon that an image is disarranged due to transfer is also apt to be produced.
Further, in the disclosed technique (e), a large quantity of transparent and colorless toner stays behind after a toner image is transferred. Unless the transparent and colorless toner is cleaned and removed before a next image is formed, the surface of the image carrier cannot be made uniform. It is therefore necessary to recover and abolish the transparent and colorless toner by use of a cleaning unit. Thus, the problem to reduce waste toner cannot be solved.
As the technique (2) to reuse recovered toner, the following techniques have been proposed. In the technique disclosed in JP-A-54-121133, toner recovered by a cleaning unit is returned to a developing machine through a conveyance path so as to be reused. In the technique disclosed in JP-A-53-125027, a cleaning unit and a developing machine are integrated into a unit, and toner recovered by the cleaning unit is dropped or conveyed into a storage chamber where toner for use in development is accommodated. Further, a technique in which residual toner on an image carrier is recovered by a developing machine without providing a cleaning unit is disclosed, for example, in JP-A-54-109842, JP-A-59-133573, JP-A-59-157661, or the like. In such an apparatus, when an image is developed after a previous toner image has been transferred, toner staying behind in a background portion is transferred to a developing roll in an electric field in a development area so as to be recovered.
In the above-mentioned apparatus disclosed in JP-A-54-121133, JP-A-53-125027, JP-A-54-109842, JP-A-59-133573, and JP-A-59-157661, there is no case that recovered toner is accumulated, but paper dust or the like entrained at the time of transfer or the like is also recovered in the developing machine. Thus, the paper dust may cause an image defect. In addition, the electification property fluctuates due to repeated use of toner so that the stability of image density may be damaged. Thus, it may be necessary to exchange a developer accommodated in the developing machine for a new one, and abolish the old toner. Further, when a special device is used for conveying toner to the developing machine, the structure becomes complicated.
The above-mentioned technique (3) in which the cleaning step is not carried out is disclosed, for example, in JP-A-3-172880. Generally, unless an image carrier is cleaned after transfer, there arises a problem such as a positive ghost in which residual toner is printed out in the next image forming step, or a negative ghost caused by the light shielding effect of the residual toner. In the above-mentioned technique disclosed in JP-A-3-172880, the transfer efficiency of toner is increased to 80% or more so that such a ghost is prevented from being produced. On the other hand, JP-A-3-114063 discloses that the quantity of residual toner is made not larger than 0.35 mg/cm2 so that the production of the ghost can be avoided. Further, another publication discloses a technique in which residual toner after transfer is disturbed with a brush or the like so as to prevent the residual toner from producing any ghost. However, to prevent the production of a ghost or the production of fog satisfactorily in such an apparatus, it is necessary to improve the transfer efficiency likewise.
Thus, as techniques which can solve the problems belonging to the techniques (1) to (3) all at once, there are proposed some methods for forming particulates or a high-releasable coating layer on the surface of an image carrier as follows.
JP-A-57-23975 discloses a technique for giving treatment to the surface of an image carrier so as to coat the image carrier with high-releasable silicon. In addition, JP-A-57-8569 discloses a technique for giving fluoropolymer treatment to the surface of an image carrier.
In addition, JP-A-7-234592 discloses a technique in which an elastic layer is provided on the surface of intermediate transfer means while microparticles the particle size of which is not larger than half of the grain size of toner is firmly fixed to the surface of the intermediate transfer means.
Further, U.S. Pat. No. 5,666,193 or JP-A-9-230717 discloses a technique in which particulates each having a volume-weighted diameter smaller than about 3 microns are partially planted in an elastic layer on the surface of an intermediate transfer member.
Furthermore, JP-A-9-212010 discloses a technique in which particulates each having a diameter smaller than that of toner are made to adhere substantially uniformly to the surface of an image carrier, and the toner is transferred onto the particulates.
Further, JP-A-2000-206801 discloses a technique in which an intermediate transferor has an inorganic coating layer.
However, the above-mentioned conventional techniques have problems as follows. That is, in the technique disclosed in JP-A-57-23975 or JP-A-57-8569, coating treatment with high-releasable silicon is given to the surface of the image carrier, or fluoropolymer treatment is given to the surface of the image carrier. In such a technique, the initial transfer rate cannot be improved beyond about 95%, and there is a limit in the improvement of the transfer rate. In addition, there is a problem that the surface subjected to silicon coating or fluoropolymer treatment is contaminated with the passage of time so that the transfer rate is degraded gradually.
On the other hand, in the technique disclosed in JP-A-7-234592, the initial transfer rate is sufficiently acceptable. However, there is a problem that the microparticles firmly fixed to the surface of the intermediate transfer means are detached with the passage of time so that the transfer rate cannot be kept for a long time.
Further, in the technique disclosed in U.S. Pat. No. 5,666,193 or JP-A-9-230717, it is necessary to plant particulates into the elastic layer on the surface of the intermediate transfer member surely. Actually, however, it is difficult to plant the particulates into the elastic layer on the surface of the intermediate transfer member surely. In addition, in the same manner as in the technique disclosed in JP-A-7-234592, the planted particulates are detached with the passage of time so that both the initial transfer rate and the tenability of the transfer rate are insufficient.
Further, in the technique disclosed in JP-A-9-212010, particulates each having a diameter smaller than that of toner are made to adhere substantially uniformly to the surface of the image carrier, and the toner is transferred onto the particulates. The initial transfer rate is sufficiently acceptable. However, there is a problem that the particulates adhering to the surface of the image carrier are detached due to mechanical stress or removed due to the adhesive force with the toner, so that the tenability of the transfer rate is insufficient.
Further, in the technique disclosed in JP-A-2000-206801, the intermediate transferor has an inorganic coating layer. In the same manner as in JP-A-57-23975 or JP-A-57-8569, the initial transfer rate cannot be improved beyond about 95%. Thus, there is a problem that there is a limit in the improvement of the transfer rate.
As described above, to reduce toner to be recovered/abolished to the utmost, or to omit abolishment/disposal of toner, it is necessary to make the transfer efficiency and the tenability thereof better than those in the conventional techniques. In addition, to eradicate toner to be abolished, it is necessary to improve the transfer efficiency dramatically enough to prevent an image defect such as a ghost, fog, or the like, from being produced even if residual toner after transfer is not cleaned.
Thus, the present invention was developed to solve the foregoing problems belonging to the conventional techniques. It is an object of the present invention to provide an image carrier, and an image recording apparatus and an image recording method using the image carrier, in which the transfer efficiency can be improved on a large scale when a toner image is transferred to a recording sheet or an intermediate transferor so as to reduce toner to be recovered/abolished; or in addition, a cleaning unit can be dispensed with or simplified in accordance with necessity; production of waste toner can be prevented to the utmost; and high transfer efficiency is further kept stably for a long time.
To solve the foregoing problems, the image carrier, and the image apparatus and the image recording method using the image carrier according to the present invention has a feature that the surface of the image carrier for carrying a toner image is formed of a material having a high affinity to particulates the particle size of which is smaller than that of toner and the surfaces of which have been treated to be hydrophobic, and a layer of the particulates treated to be hydrophobic is provided on the surface of the image carrier.
Toner for visualizing a latent image is transferred onto the particulate layer. Thus, a toner image is formed in the state where the toner is laminated on the particulate layer. Generally, toner adheres to the image carrier by electrostatic force (occasionally sucked magnetically), but non-electrostatic adhesive force such as van der Waals force, or the like, also acts on the toner. However, if a toner image is formed on the particulate layer as described above, it is possible to form a gap between toner grains and the image carrier, or it is possible to reduce the contact area between the toner grains and the image carrier. Thus, non-electrostatic adhesive force such as van der Waals force, or the like, is reduced. Accordingly, if an electric field acts on the toner grains at the time of transfer, the toner grains are transferred easily by electrostatic force so that the transfer can be performed with high efficiency close to 100%.
To allow such a mechanism to last for a long time, it is presupposed that there is a particulate layer between the toner grains and the image carrier. It is therefore necessary to retain the particulate layer on the surface of the image carrier surely.
Therefore, according to the present invention, the surface of the image carrier is formed of a material having a high affinity to particulates the particle size of which is smaller than that of toner and the surfaces of which have been treated to be hydrophobic, and a layer of the particulates treated to be hydrophobic is retained on the surface of the image carrier.
As the material having a high affinity to the particulates treated to be hydrophobic, for example, there is a hydrophobic treatment agent. Active groups such as xe2x80x94Cxe2x95x90O, xe2x80x94OH, xe2x80x94COOH, etc. on the surface of the image carrier, generally, react easily with the hydrophobic treatment agent having a coupling function with such active groups, so as to form hydrophobic groups on the surface of the image carrier. On the other hand, the surfaces of the particulates are also treated to be hydrophobic so that there are hydrophobic groups also on the surfaces of the particulates. As a result, the hydrophobic groups having a high affinity attract each other due to van der Waals force or the like. Thus, it is conceived that the particulates can be retained on the surface of the image carrier surely.
As another form showing such an affinity, there is an SP (Solubility Parameter) value. It has been proved that materials having SP values close to each other have a high affinity to each other. In addition, it has been proved that the hydrophobic agent of the particulates and the material provided on the image carrier come into tight contact with each other if the difference between the SP value of the hydrophobic agent of the particulates and the SP value of the material of the image carrier is not larger than 1.1 so that the transfer rate and the tenability thereof are superior.
As another form of such a material having a high affinity to the particulates, there is a material which is boiled at a temperature of 150xc2x0 C. or lower, or which is hydrolyzed at a temperature of 150xc2x0 C. or lower. By use of such a material, surface treatment can be given to the image carrier at a comparatively low temperature of 100xc2x0 or lower, including a room temperature. Thus, it is possible to prevent thermal deformation of a rubber material, interface peeling of a member having a multi-layer structure, or the like, due to heating in the surface treatment step. Such interface peeling is caused by a difference in thermal expansion coefficient among constituent materials of the multi-layer member. As a result, it is possible to give treatment to any material constituting the image carrier or the intermediate transferor.
Examples of materials satisfying such properties may include hexamethyldisilazane (SP value 6.5, boiling point 126xc2x0 C.), trimethylmethoxysilane (SP value 6.7, boiling point 57xc2x0 C.), methyltrimethoxysilane (SP value 7.3, boiling point 102xc2x0 C.), triethylchlorosilane (SP value 7.8, boiling point 145xc2x0 C.), trimethylbromosilane (SP value 7.8, boiling point 80xc2x0 C.), etc.
Next, description will be made about a surface treatment method with hexamethyldisilazane by way of example.
Spherical silica powder (particulates) is surface-treated with hexamethyldisilazane in advance, so as to have (CH3)3Sixe2x80x94 groups on the surface and have an average particle size of 150 nm. One part by weight of the silica powder is mixed and stirred into 100 parts by weight of a hexamethyldisilazane solvent. The silica powder and the hexamethyldisilazane solvent have (CH3)3Sixe2x80x94 groups in common. Therefore, the silica powder and the hexamethyldisilazane solvent have a high compatibility to each other so that they disperse easily. Thus, no precipitate is produced, and the work of solution preparation and application is easy.
This solution is applied onto an intermediate transfer drum as an intermediate transferor by use of a roll coater, air-dried at a temperature of 22xc2x0 C. and a humidity of 55% for 5 hours, and further heat-treated in a drying furnace at 100xc2x0 C. for 1 hour. During such air-drying and heat-treating, hexamethyldisilazane as a solvent reacts with moisture in the air so as to be hydrolyzed, and reacts with active groups contained in the surface layer material on the intermediate transfer drum so as to form (CH3)3Sixe2x80x94 groups on the surface layer. Thus, the silicon powder and the intermediate transferor surface have common chemical species even in the solid-layer state as well as in the solution state. It is therefore possible to form a thin layer with high adhesion property.
Although the above description was made in the case where hexamethyldisilazane was used as a solvent for silicon powder, the treatment agent for the silicon powder and the solvent therefor do not have to be the same species. For example, even if a solution in which the above-mentioned silicon powder is dispersed into a solvent of triethylchlorosilane is used as coating, similar effects can be obtained. Various combinations can be made.
Further, as another form, the above-mentioned high-affinity material does not have to be formed later on the image carrier surface. That is, arrangement may be made so that the material forming the surface of the image carrier have a low affinity to the high-affinity material and the high-affinity material is dispersed into the surface material of the image carrier. With such an arrangement, a so-call bleed phenomenon occurs so that the high-affinity material runs out on the surface of the image carrier gradually. Thus, it is possible to obtain a similar effect to that in the case where the high-affinity material is applied to the surface of the image carrier.
Description will be further made about this example. For example, in an intermediate transfer drum using a mixture of acrylic resin and silicon resin as a surface layer material, KPN3504 or KBM3103 and further KBM7103 made by Shin-Etsu Chemical Co., Ltd. are added into the surface layer material in advance. Such a solution is applied by a roll coating method or the like so as to form a surface layer. The above-mentioned additives shift to the surface of the surface layer in a coating film forming step (heat treatment step) because the additives are low molecular weight components. When the above-mentioned solution in which silicon powder surface-treated with hexamethyldisilazane is dispersed into hexamethyldisilazane is applied to the intermediate transfer drum in which the additives have shifted to the surface, the additives added in advance react with hexamethyldisilazane so as to forma firmer surface-treated layer. Incidentally, KPN3504 made by Shin-Etsu Chemical Co., Ltd. is an aminosilane mixture, KBM3103 likewise is decyltrimethoxysilane, and KNM7103 likewise is xcex3-mercaptopropyltrimethoxysilane.
Incidentally, all the toner grains to be transferred onto the image carrier to which the particulates adhere substantially uniformly do not have to be always transferred onto the particulates adhering to the image carrier. If most of the toner grains are transferred onto the particulates, it is still possible to obtain a transfer effect substantially as high as that in the case where all the toner grains are transferred onto the particulates, even though a part of the toner grains are transferred directly onto the image carrier.
In addition, when the image recording apparatus is an apparatus in which an image carrier having a photosensitive layer is irradiated with image light so as to form a latent image, or when the image recording method is to irradiate an image carrier having a photosensitive layer with image light so as to form a latent image, the particulates may be composed of a light transmissive material. In this case, after a uniform particulate layer is formed on the surface of the image carrier, if the surface of the image carrier is charged uniformly and irradiated with image light from above the particulate layer, an accurate latent image can be formed. Thus, the image visualized by the adhesion of the toner becomes clear. In this case, the light transmission property is involved in effective transmission property with respect to light of wavelength in service for image exposure. That is, if the particle size of the particulates is not larger than xc2xd of the wavelength of light for image exposure, the particulates do not cause an obstacle to exposure even if the particulates are opaque in view of its material structure. Such particulates are included in the light transmissive particulates referred to in the present invention.
The particulates on the image carrier may be transferred uniformly in advance before the image recording apparatus is put into service, or the particulates may be supplied onto the image carrier by particulate supply means in an early stage when the image recording apparatus is put into service. In either case, at least the surface of the image carrier has to be beforehand coated with a material having a high affinity to the particulates treated to be hydrophobic.
In addition, it is preferable that the particulate supply means can supplement particulates transferred from the image carrier to a recording medium or the like together with the toner, appropriately, so that the particulates adhere onto the image carrier substantially uniformly whenever an image is formed. As the means for transferring the particulates to the image carrier, various forms can be adopted. For example, there may be adopted means which has the same configuration as the developing machine and which accommodates powder particulates in place of colored toner, or means which accommodates a developer including both toner and powder and which also functions as both the particulate supply means and the developing machine. This means which functions as both the particulate supply means and the developing machine may transfer the powder uniformly in the state where no latent image is formed, before forming a latent image and transferring the toner. Alternatively, the means may transfer the particulates included in the developer onto the latent image carrier together with the toner simultaneously so as to form or keep a uniform layer of the particulates. On the other hand, as a method for making the particulates adhere to the surface of the image carrier electrically, other than the above-mentioned means having the same configuration as the developing machine, there is a method in which particulates are dispersed like cloud and made to adhere to the surface of the image carrier by the force of an electric field. As means for dispersing the particulates like cloud and making the particulates adhere to the surface of the image carrier, for example, there is a method using mechanical vibration, the air, an ultrasonic wave, or an alternating electric field, or a method in which particulates are made to adhere to a body like a roll, a brush, a web, or a paintbrush, and the body with the particulates is rotated, vibrated or moved. Alternatively, the powder may be rubbed mechanically by a rotary brush, a magnetic brush in which granulates are connected to be spicate by magnetism, a roll formed of a flexible elastic body, felt, a paintbrush, or the like. Such a mechanical method may be used together with the above-mentioned electric method.
Particulate smoothing means is prepared separately from the particulate supply means. The particulates supplied from the particulate supply means onto the image carrier are not always uniform. In addition, the particulates have a tendency to accumulate with the passage of time. In such a case, the particulate smoothing means is effective. As the particulate smoothing means, various forms may be adopted. A rotatable brush, a rotatable roll, an endless belt, or the like, is preferable. Particularly, it is preferable that the rotatable brush, the rotatable roll or the endless belt is conductive with a volume resistivity in a range of from 106 xcexa9cm to 1013 xcexa9cm, and a bias voltage in a range of from xe2x88x921,000 V to +1,000 V is applied to the rotatable brush, the rotatable roll or the endless belt, while the rotatable brush, the rotatable roll or the endless belt has a difference in velocity from the image carrier. With such a configuration, there is an effect that flocculates of particulates supplied onto the image carrier later are disentangled to be smoothed. Although the mechanism is not quite understood, even if there is a portion where the particulates on the image carrier are detached, particulates are supplied to that portion, and the material having a high affinity to the particulates on the image carrier attracts the particulates by priority. Thus, there is an effect that the particulate layer is repaired easily.
Alternatively, another means is used also as the particulate smoothing means without providing any special particulate smoothing means. As apparatus or steps which can be used also as the particulate smoothing means, there are apparatus for cleaning, temporarily cleaning, contact-charging, contact-transferring, and so on. In each case, the apparatus can be used as a particulate smoother at the same time as or at a timing different from its original step. On these occasions, it is obvious from the above description that a velocity difference from the image carrier is effective in each case. Further, if there is provided a velocity difference between image carriers, for example, between a photosensitive body and an intermediate transferor or the like, the image carriers function as particulate smoothers in the same manner. Using another member also as the particulate smoothing means has an immeasurable merit on the simplification of the apparatus configuration.
As another form of the particulate smoothing means, there is a method in which abrasive particulates are supplied separately from the above-mentioned particulates. As the abrasive particulates, ones smaller in size than the toner can be used in the same manner as the above-mentioned particulates. However, it is preferable that the shapes of the abrasive particulates are undefined or the surfaces thereof are not smooth. The above-mentioned particulate smoothing means can be used also as means for supplying the abrasive particulates. However, the abrasive particulates have smoothing ability so high that the surface of the image forming means is damaged in excess supply of the abrasive particulates. It is therefore preferable that the abrasive particulates are usually supplied at a weight ratio in a range of from 0.01% to 1.0% with respect to the toner. As the method for supplying the abrasive particulates, independent supply means may be provided, or the abrasive particulates may be contained in the developer, in the same manner as the above-mentioned particulates.
As for the properties of the above-mentioned particulates, it is preferable that the volume resistivity is set to be in a range of from 1xc3x97108 xcexa9cm to 1xc3x971014 xcexa9cm. By use of the particulates satisfying this condition, even if the adhesive force between the image carrier and the particulates is lowered, it is possible to reduce the quantity of the particulates electrostatically transferred from the image carrier to a recording medium or the like. Thus, the tenability of the particulate layer is improved so that excellent transfer properties can be obtained stably. The reason whey such an excellent result can be obtained will be described as follows. The above-mentioned particulates are interposed between the toner and the image carrier so as to reduce the adhesive force therebetween. Thus, the particulates have an effect to improve the transfer efficiency. After a toner image is transferred, most of the particulates themselves remain on the image carrier. However, a part of the particulates are transferred together with the toner when the toner image is transferred. There is a correlation between such a tendency and the volume resistivity of the particulates made to adhere to the image carrier. That is, particulates high in volume resistivity are apt to be transferred together with the toner when the toner image is transferred. On the contrary, particulates low in volume resistivity are apt to remain on the image carrier. This is due to the electrification properties of the particulates. If the volume resistivity of the particulates adhering to the surface of the image carrier is high, the particulates are charged easily in the step where the image carrier is charged. For example, in the case where the electrification polarity of the image carrier is minus, the particulates are charged to be minus. At this time, the potential caused by the charges of the particulates is lower than the potential of the image carrier. However, it reaches several percentages to several tens of percentages of the potential of the image carrier because of some volume resistivity. When the particulates charged to the same polarity as the electrification polarity of the image carrier in such a manner suffer a transfer electric field of the reverse polarity in the transfer step, a part of the particulates are transferred together with the toner by electrostatic force (Coulomb force). Therefore, if the volume resistivity of the particulates is low, that is, if the volume resistivity is lower than about 1xc3x971014 xcexa9xc2x7cm, unnecessary charging is prevented so that the particulates are prevented from being transferred from the surface of the image carrier by Coulomb force at the time of transfer. Thus, the particulate layer on the image carrier can be retained. On the contrary, if the volume resistivity of the particulates is lower than about 1xc3x97108 xcexa9xc2x7cm, charge transfer occurs through the particulates on the image carrier. Thus, if the image carrier is a photosensitive body, there arises a defect that an electrostatic latent image becomes unclear (an image is blurred).
It is preferable that the thickness of the particulates on the image carrier is not larger than 3 xcexcm, and the particulate layer is formed from about 1 to 5 layers. If the thickness increases, the particulate layer is apt to be uneven to cause transfer density unevenness. Even if an even layer can be formed, a transfer electric field is weakened by the particulate layer so as to cause the lowering of the transfer rate.
On the contrary to those of the abrasive particulates, spherical or smooth-surface convex shapes can be adopted as the shapes of the particulates. This is because such shapes can make a uniform particulate layer easily, that is, the contact between the image carrier and the particulates becomes uniform easily, and the contact between the formed particulate layer and the toner also becomes uniform easily so that the transfer rate becomes uniform.
For the particulate layer to improve the transfer rate, any toner shape and any toner grain size may be used. However, spherical toner is more preferable. The spherical toner is so low in toner adhesive force and so uniform that the transfer rate is very high. Thus, a cleaner can be omitted.
As another form for making the present invention more effective in service, the surface of the image carrier is formed of a material having a high affinity to particulates treated to be hydrophobic, and the contact angle of the surface of the image carrier having the particulates treated to be hydrophobic is set to be not larger than 100xc2x0, preferably not larger than 90xc2x0, with respect to pure water. In a conventional system, for example, in JP-A-8-152786, to make it possible to omit a cleaning unit, it is necessary to make the contact angle of the surface of a photosensitive body not smaller than 90xc2x0. The large contact angle results in the reduction of surface energy. Thus, it becomes difficult for toner to adhere. As a result, the transfer rate is improved. This sounds logical. On the other hand, according to the acute investigation of the present inventor, it was proved that if the contact angle was not smaller than 100xc2x0, the initial transfer rate was indeed excellent, but the tenability thereof was not excellent, though the reason was not known well. On the contrary, it was proved that if the contact angle was not larger than 100xc2x0, both the initial transfer rate and the tenability thereof were very excellent, and particularly if the contact angle was not larger than 90xc2x0, they became especially excellent. It is estimated that when the image carrier has a contact angle not smaller than 100xc2x0, the adhesive force of the particulates to the image carrier is weak to be easily detached. As the image carrier according to the present invention, there can be used one in which particulates treated to be hydrophobic are provided on the surface of the image carrier, and the contact angle of the image carrier having the particulates on the surface is not larger than 100xc2x0, preferably not larger than 90xc2x0.
As another form for making the present invention more effective in service, the surface of the image carrier which has not yet been coated with the material having an affinity to the particulates has a resin layer, and the substrate of the image carrier has elasticity. The material having an affinity to the particulates are fixed more firmly and more stably so that the tenability of the transfer rate becomes more excellent. In this case, the substrate of the image carrier may be set to be not larger than 30xc2x0 in JISA hardness. The adhesion between the toner and the image carrier is improved. Thus, the transfer efficiency is improved not only when the toner is transferred to the image carrier, but also when the toner is transferred from the image carrier (for example, when the toner is transferred from a photosensitive body as an intermediate transferor). In addition, a similar effect can be obtained when the microhardness of the substrate of the image carrier is in a range of from 20xc2x0 to 60xc2x0.
Here, a microhardness meter for measuring the microhardness is made up for measuring hardness of small, thin and soft rubber parts which could not be measured by a conventional ASKER-C rubber hardness meter or the like. A measured value of the microhardness meter has hardness information in the vicinity of the surface of a microscopic measuring portion in comparison with a conventional hardness measured value. As such a microhardness meter of this type, there is a xe2x80x9cMicro durometer (type MD-1)xe2x80x9d made by KOBUNSHI KEIKI CO., LTD. The MD-1 durometer is used as the microhardness meter in the present invention.